For high-speed and large capacity data transmission, optical communication has been put into practical applications in the backbone of long-distance communications networks. Optical communication has also been applied to the field of information processing systems including computers or servers for connecting equipment. Introducing optical signals inside equipment or boards is now being pictured.
Meanwhile, silicon photonics, a technique for fabricating minute optical circuits using silicon is attracting attention. Silicon is transparent to infrared light and has a large index of refraction (about 3.5). Fine processed products are easily mass-produced using a semiconductor fabrication process. Making use of the large index of refraction of silicon, light is confined in a silicon wire waveguide. An optical transceiver and a wavelength multiplexer/demultiplexer are integrated in a chip of several millimeters square in fabrication of a wavelength division multiplexing (WDM) transceiver.
Silicon photonics allows a number of chips to be fabricated on a wafer at a time, as per in a semiconductor process. If optical chips are mass-produced, quality check is needed for these chips. In the ordinary semiconductor process, wafer testing is carried out to inspect the chips and only those chips with satisfactory qualities are selected. It is efficient for silicon photonics to employ the same approach for chip inspection.
In the semiconductor process, an electric probe is made to touch an electric terminal of the chip to test the performance. In silicon photonics, a test light has to be input to and output from a chip fabricated on the wafer. In order to take the light from the silicon wire waveguide on the wafer, two approaches are knows. One technique is to use a spot size converter (SSC). See, for example, T, Watanabe, et al, “Si wire waveguide devices”, Proceedings of SPIE, Vol. 6775, 67750K (September 2007). The other technique is to use a grating coupler (GC). See, for example, Japanese Patent Application Laid-open Publication No. 2011-107384 A.
A spot size converter expands the mode field diameter (MFD) of the light propagating through the silicon wire waveguide by providing an inverse-taper core and covering the inverse-taper core with a large core of a second waveguide. However, because the spot size converter is arranged at an edge of the chip to input and output light signals to and from the chip, testing or inspection cannot be performed until the wafer is cut into chips.
With a grating coupler, chip testing can be performed on the wafer because the light with an expanded mode field diameter is reflected upward by the diffraction grating after propagation through the inverse-taper core. However, the emission angle of the light is defined by the wavelength and the pitch of the diffraction grating, and the operative wavelength band is narrow. For this reason, the configuration with a grating coupler is unsuitable for WDM with a wide wavelength band. Besides, a single mode optical fiber (SMF) has to be aligned to the grating coupler for testing the chip to an accuracy of micrometers. It is impractical to inspect a number of chips on the wafer using the grating coupler configuration. Another problem in this approach is large polarization dependence.
As yet another approach, a structure for rotating the direction of polarization of light in the optical waveguide is known. See, for example, Japanese Patent Application Laid-open Publication No. 2010-88110 A.
Optical waveguides used in fiber-optic communication are generally designed such that transverse electric (TE) mode and transverse magnetic (TM) mode make as little difference as possible. However, in silicon photonics, the difference between the TE mode and the TM mode becomes conspicuous because light is confined in a microscopic core. Accordingly, silicon photonic wire waveguides are designed so as to operate with only one of the polarized states (generally, in the TE mode).
As long as an optical circuit is closed in a silicon photonic chip, polarization does not become a serious problem. However, when the silicon photonic optical circuit is connected to light signals input from or output to an optical fiber, the polarization issue arises. In an ordinary single-mode optical fiber (SMF), the polarization state of the propagating light is not maintained and both the TE mode and the TM mode exist in the incident light from the SMF on a silicon photonic chip. The TE component and the TM component are separated using a polarization diversity scheme, and the respective polarization states are treated separately.
A polarization-maintaining optical fiber (PMF) is able to maintain the TE-to-TM ratio. By connecting a silicon photonic chip to another silicon photonic chip using a PMF, only TE-polarized light can be received at the receiving end. In order to realize this, the orientation of the polarization axis of the PMF has to be aligned. Specifically, the direction of rotation of the polarization axis is adjusted, while monitoring the microscopic image of the end face of the PMF. This method is complicated and costly, and it is unrealistic.
It is desired to provide a technique for carrying out inspection of optically interconnected chips on a wafer without using a complicated structure, while taking the polarization state of light in an optical fiber into account.